1. Field of the Invention
The present invention relates to a fluoro material which may be used as an electrode active material, and also to a process for producing it.
2. Description of Related Art
Lithium batteries are known which use an insertion compound as operating basis for the positive electrode, such as LixCoO2, 0.4≦x≦1, which is used neat or as a solid solution with nickel and manganese and aluminum. The main barriers to the spread of this type of electrochemistry are the scarcity of cobalt and the excessively positive potential of the transition metal oxides, the consequences of which are safety problems for the battery.
LixTMmZyP1-sSisO4 compounds (“oxyanions”) are also known, in which TM is chosen from Fe, M and Co, and Z represents one or more elements which have a valency of between 1 and 5 and which can be replaced in the sites of the transition metals or of the lithium. These compounds exchange only lithium, and have only very low electron and ionic conductivity. These handicaps may be overcome by using very fine particles (such as nanoparticles) and by depositing a carbon coat by pyrolysis of organic compounds. The drawbacks associated with the use of nanoparticles are low compactness, which is reflected by a loss of specific energy, and this problem is further exacerbated by the deposition of carbon. Furthermore, the deposition of carbon takes place at high temperature, under reductive conditions. In practice, it is difficult to use transition elements other than FeII and MnII, since the elements CoII and NiII are readily reduced to the metal state. This is likewise the case for FeIII, MnIII, CrIII, VIII and VIV, which are advantageous dopants for increasing the ionic or electron conductivity.
Other compounds have been proposed, especially compounds corresponding to the general formula AaMb(SO4)cZd in which A represents at least one alkali metal, Z represents at least one element chosen from F and OH, and M represents at least one divalent or trivalent metal cation. L. Sebastian, et al., [J. Mater. Chem. 2002, 374-377] describe the preparation of LiMgSO4F via a ceramic route, and also the crystallographic structure of said compound, which is isotypic of the structure of tavorite LiFeOHPO4. The authors mention the high ionic conduction of this compound, and suggest that the LiMSO4F compounds in which M is Fe, Co or Ni, which would be isostructural, appear to be important for the insertion/redox extraction of lithium involving MII/MIII oxidation states. The authors also state that the preparation of Fe, Ni or Co compounds via a ceramic route is underway, but no subsequent publication regarding these compounds has been made.
In addition, US 2005/0 163 699 describes the ceramic preparation of the abovementioned compounds AaMb(SO4)cZd. The technique is illustrated by concrete examples concerning compounds in which M is Ni, Fe, Co, Mn (Mn+Mg), (Fe+Zn) or (Fe+Co). These compounds are prepared ceramically from LiF, which is the Li precursor, and from the sulfate of the element or elements M. Among these compounds, the most advantageous are compounds which contain Fe, since, besides their relatively low cost, they are capable on the basis of structural and chemical considerations (especially the ionocovalence of the bonds) of having advantageous electrochemical properties in a desirable potential range for ensuring reliable use for large-volume applications. For reasons of inductive effect, sulfates should have higher potentials than phosphates, irrespective of their structure. Examples of preparation of compounds containing various metal elements are described, but no electrochemical property is reported. Thus, example 2 describes the preparation of the compound LiFeSO4F via a ceramic method at 600° C., which gives an inhomogenous compound, and then 500° C. in which the compound is red-black, or else at 400° C. in air, in which the compound is red. This method is capable of enabling the reduction of the SO42− group via Fe2+ in the absence of oxygen according to: SO42++Fe2+SO2+2O2−2Fe3+. The red color observed in the compounds obtained at the various temperatures is due to the O2−/Fe3+ combination in a crystal lattice such as the oxide Fe2O3. It is moreover known that FeII compounds become oxidized in air from 200° C., giving FeIII, and the preparation of example 2 at 400° C. in air confirms this. The compounds containing iron which are prepared ceramically from LiF and iron sulfate according to US-2005/0 163 699 therefore do not consist of LiFeSO4F. Similarly, it appears that the compounds in which M is Co or Ni are unstable at the temperatures used during the recommended ceramic-route preparation. It is therefore implausible that the compounds described in US-2005/0 163 699 were really obtained.
WO 2010/00466610 describes a process for preparing compounds LiMSO4F in which M represents one or more transition metals, in particular Fe partially replaced with Mn. These compounds are obtained via ionothermal synthesis starting with LiF and a hydrated (preferably monohydrate) M sulfate which has a structure similar to that of tavorite, in terms of arrangements of octahedra and tetrahedra. Their structure is similar to that of the precursor sulfate. These materials may be used as cathode active material and operate at a potential of about 3.6 V.
The performance qualities of a lithium battery depend especially on the redox potential of the cathode active material. In particular, the energy density delivered by the battery is higher if the redox potential of the cathode active material is higher, all things being otherwise equal.
The aim of the present invention is consequently to propose a novel material that is useful as a cathode active material in a lithium battery, and also a process for manufacturing it.